Need of flat panel display for computer rises accompanied with spreading of information terminals. Moreover, electronic paper or digital paper as a thin, light and easily mobile displaying medium is needed because the chance of providing information in a form of electronic signals instead of paper medium is increased accompanied with the progress of information system.
In the planar displaying apparatus, the displaying medium is generally constituted by the use of an element applying liquid crystal, organic EL or electrophoresis. In such the displaying medium, technology in which an active driving element (TFT element) is principally applied for obtaining a uniformity of the brightness and a high rewrite speed of the image. In usual displays for computer, for example, the TFT elements are formed on a glass substrate and the liquid crystals or the organic EL elements are sealed.
A semiconductor such as a-Si (amorphous silicon) and p-Si (polysilicon) is employable for the TFT element. The TFT element is formed by successively laminating the silicon semiconductor and a metal layer according to necessity for forming a source electrode, a drain electrode and a gate electrode. A production process utilizing vacuum such as a spattering process is usually needed for producing the TFT element.
In the production of the TFT element, the production process including a vacuum process using a vacuum chamber has to be repeatedly applied for forming the constituting layers. Consequently, the costs for equipment and running the production become very high. For the TFT element, processes such as a vacuum deposition, doping and photolithography should be repeatedly performed for each of the layers. Therefore, the element is formed on the substrate through several tens processes. In a semiconductor portion making the important point of switching action, plural kinds of layer such as a p-type semiconductor layer and an n-type semiconductor layer are laminated. In such the processes for producing the silicon semiconductor, the change of the equipment corresponding to the requirement of large-sizing of the displaying image is difficult because largely changing in the design of the production apparatus such as the vacuum chamber is necessary.
The substrate material is desired to be a material capable of withstanding a process temperature since the conventional production processes for TFT elements employing silicon include a high temperature process.
Consequently, glass is only practically usable. Therefore, the displaying apparatus becomes one which is heavy, lacking in the flexibility and easily broken by falling when the displaying apparatus is constituted by the usual TFT elements. Such the properties caused by forming the TFT elements on the glass substrate are not suitable for satisfying the requirements for the light mobile thin display accompanied with the progress of the information system.
Besides, in recent years, studies on organic semiconductor material having high charge transport ability have intensively been done. Such the compounds are expected as an element of an organic laser oscillation element (Non-patent Document 1, for example) additionally to the charge transport material of the organic EL element, and that of organic thin film transistor such as those reported in various reports (Non-patent Document 2, for example).
Though it may impossible to produce the semiconductor element by such the low temperature process by using the usual silicon type semiconductor material, it may be possible with respect to the device employing the organic semiconductor. Therefore, the limitation of the substrate heat resistance is alleviated and, for example, the TFT element may be possible to be formed on a transparent resin substrate plate. When the TET elements can be formed on the transparent resin substrate plate and the displaying materials can be driven by the TFT elements, the display may be produced as one lighter in weight, and higher in flexibility than those of conventional ones, and be hardly or difficulty broken by falling.
However, the organic semiconductors studied for realizing such the TFT element are only limited kinds of compound, for example, an acene-type compound such as pentacene and anthracene (Patent Document 1, for example), a phthalocyanine including lead phthalocyanine, a low molecular weight compound such as perylene and its tetracarboxylic acid derivative (Patent Document 2, for example), an aromatic oligomer typically such as a hexamer of thiophene so-called as α-thienyl or sexithiophnen (Patent Document 3, for example), a compound of naphthalene or anthracene symmetrically condensed with a 5-member aromatic heterocycle (Patent Document 4, for example), a mono-, oligo- and poly-thienylenopyridine (Patent Document 5, for example) and a conjugate polymer such as polythiophene, poly(thienylene vinylene) and poly(p-phenylene vinylene) (Non-patent Documents 1-3, for example). Thus, the development of a semiconducting composition material employing a new charge transport material exhibiting high carrier mobility has been demanded.
Cost reduction is expected in view of productivity when a pentacene derivative-containing layer about which excellent properties as a commonly known organic semiconductor material are reported can not be formed via a vacuum evaporation process, but be formed via solution coating such as an ink-jet method and so forth. However, There is a manufacturing problem pointed out, such that stability of the coating solution is insufficient to the fact that the pentacene derivative had insufficient solubility to various organic solvents, and crystallization or precipitation also tends to be generated after preparing a solution. Thus, disclosed is a technique concerning an organic TFT material with the pentacene derivative into which substituents were introduced (refer to Patent Documents 9 and 10, for example) in order to improve solubility as well as stability of the coating solution, but there is a problem such that it is difficult to obtain sufficient solubility by introducing an alkyl group and the like, to maintain operability in temperature control during coating, and to prepare an evenly coated layer.
Japanese Patent O.P.I. Publication No. 2003-292588, U.S. Patent Opened Application Nos. 2003/13658, 2003/160230 and 2003/164495 describe as follows. “The mechanical strength of a logical integrated circuit element for microelectronics is largely improved and the using life time of it can be prolonged by employing a polymer TFT. However, the polythiophene semiconductors are mostly instable in air because which is doped by oxygen in the atmosphere so that the electro conductance of it is increased. As a result of that, the off current of the device produced from such the materials is increased, and the on/off ratio of the electric current is decreased. Japanese Patent O.P.I. Publication No. 2003-292588, U.S. Patent Opened Application Nos. 2003/13658, 2003/160230 and 2003/164495 describe as follows. “The mechanical strength of a logical integrated circuit element for microelectronics is largely improved and the using life time of it can be prolonged by employing a polymer TFT. However, the polythiophene semiconductors are mostly instable in air because which is doped by oxygen in the atmosphere so that the electro conductance of it is increased. As a result of that, the off current of the device produced from such the materials is increased, and the on/off ratio of the electric current is decreased. Therefore, the materials should be strictly controlled in such a way that oxygen in the atmosphere is eliminated during material processing and device production so as to prevent or minimize the oxidation doping. An advantage of TFT, in which a polymer semiconductor is utilized in a cost-effective technology in place of an amorphous silicon technology for a large-area device, is degraded since the precautionary measure raises the production cost. However, those together with other drawbacks are avoided or minimized in the embodiments of the present invention. Accordingly, it is described in Patent Documents 6-8 that an electronic device exhibiting a high resistance against oxygen as well as a relatively high ON/OFF ratio of the electric current is demanded, and the various means for solving the problems are disclosed. However, further improvement is still desired since no improvement level is sufficiently satisfied.
Patent Document 1: Japanese Patent O.P.I. Publication No. 5-55568
Patent Document 2: Japanese Patent O.P.I. Publication No. 5-190877
Patent Document 3: Japanese Patent O.P.I. Publication No. 8-264805
Patent Document 4: Japanese Patent O.P.I. Publication No. 11-195790
Patent Document 5: Japanese Patent O.P.I. Publication No. 2003-155289
Patent Document 6: Japanese Patent O.P.I. Publication No. 2003-261655
Patent Document 7: Japanese Patent O.P.I. Publication No. 2003-264327
Patent Document 8: Japanese Patent O.P.I. Publication No. 2003-268083
Patent Document 9: WO No. 03/016599
Patent Document 10: U.S. Patent Opened Application Nos. 2003/0105365
Non-patent Document 1: “Science” vol. 289, p. 599, (2000)
Non-patent Document 2: “Nature” vol. 403, p. 521, (2000)
Non-patent Document 3: “Advanced Material” 2002, No. 2, p. 99, (2002)